Multi-axis motor controls are used in industrial automation and manufacturing systems such as conveyor lines and multi-axis machine tools which require coordinated and simultaneous control of multiple motors. Currently, these systems include a main control unit coupled to an axis module unit for each motor of the system. The main control unit is programmable and includes the timing and position programming required to properly control each of the motors in the system. The axis modules are each electrically connected to a respective motor to apply power to the motor based upon control signals from the control unit. The control unit receives feedback signals from the motor. The main control unit applies control signals to the axis modules based upon the feedback signals, and the timing and position programming stored therein.
In general, the main control unit and axis modules are located in separate physical housings, and mechanically supported adjacent to each other. This permits customizing the configuration and cost of a motor control to a specific system application by limiting the number of axis modules to the number of motors required in the system. However, this arrangement requires that appropriate power and signal conductors are provided between the main control unit and the axis modules.
Several methods have been utilized in the past to provide signal routing and addressing from the main control unit to associated axis modules. The most common method is based upon a back-plane type of construction upon which the signals for a particular mechanical location of an axis module terminate in a connector having connections unique to that mechanical location. One drawback with this type of construction is that the location of a particular axis module is fixed, and connections must be keyed to prevent improper connection. Furthermore, in the back-plane type of construction, the signals are routed behind the main control unit and axis modules. This configuration requires extender boards or other arrangements to make signals accessible for testing.
As an alternative to keying the position of a module to a mechanical location and associated address, axis modules have been designed to include address switches to identify the unique position of the module. One disadvantage of address switches is that the addressing of the module must be set whenever the module position is changed during procedures such as trouble shooting, module replacement or module upgrade. This type of addressing arrangement is problematic since it is error prone and dependent on the installer to correctly set the addressing.
Another method of signal routing and addressing is based upon the use of ribbon cable connecting the main control unit to each axis module. Ribbon cable is susceptible to damage, and care must be taken in handling to prevent the need to replace the entire cable when a single module in the string requires replacement. Furthermore, since the use of a ribbon cable is typically based upon a daisy chain arrangement, depending upon the location of the cable, substantially all of the axis modules may need to be disconnected to remove a module from the middle of the chain.
Turning to the conduction of the power (i.e. 2 kW to 5 kW) from the main control unit to the axis modules to power the associated motors, a frequently used technique is to attach copper or other suitable metal strips or bars between the control unit and axis modules using mechanical pressure type connections such as screws or electrical lugs. A disadvantage of this arrangement is the need for tools to assemble and disassemble this type of power bus. Another arrangement used to conduct power from the main control module to the axis modules is discrete wiring looping from module to module with pin and socket type connectors. This type of arrangement requires that the wiring be sized and positioned to avoid conduction heating and induction losses which affect the power bus voltage. With both arrangements discussed above for power conduction, it is important to appropriately shield the power conductors when energized since these conductors typically are at an electrical potential of 360 to 480 volts.
In view of the problems involved with providing communications and power between a main motor control unit and the axis modules, it would be desirable to provide an improved communications and power bus for connecting the control unit and modules.